This invention relates to magnetic recording media or discs and in particular to a magnetic disc structure which is characterized by having excellent magnetic recording, and wear properties and stability-against-oxidation of the magnetic layer. More particularly, the invention relates to a novel combination of materials, each in a thin film form constituting a magnetic recording disc. Further, the invention relates to methods for forming magnetic materials and controlling the magnetic properties thereof so as to provide magnetic recording media having magnetic properties which are selectively variable over a desirable range in order to facilitate optimization of recording system design.
In designing a magnetic recording disc, there are a number of factors, depending upon the properties of available recording heads, that must be considered and provided for. Among the magnetic properties that are desirable, especially where high magnetic recording density is an object, is a high coercivity limited only by the write capability of the recording head. The recording system also requires that the head signal output of the recording head be greater than some minimum value. It is required in turn for thin recording films that the product of the magnetic recording film thickness and its remanent magnetization (hereinafter referred to as the magnetization-thickness product) be greater than some corresponding minimum value. In addition, the high coercivity and desired magnetization-thickness product should be accompanied by a high degree of squareness of the M-H hysteresis loop of the magnetic film. With the given values of coercivity and magnetization-thickness product established, the recording density is desirably the maximum value that is consistent with the minimum value of resolution for the recording process. Because resolution generally decreases both with increasing recording density and with an increasing value of the ratio of the magnetization-thickness product to coercivity, optimization of the recording system requires that the disc coercivity and its magnetization-thickness product be essentially the corresponding maximum and minimum values established above. Any value of magnetization-thickness product greater than its minimum value and any value of coercivity less than its maximum value necessarily results in a recording density less than what would otherwise be achievable with the recording head. Hence, there exists a need to vary the coercivity and magnetization-thickness product independently. Further, the values of these parameters need to change as improved recording heads become available and there is, therefore, an additional need for independent variability of coercivity and magnetization-thickness product over a rather large range of values. Another important constraint is that the necessary independent variability of magnetic properties be achievable on smooth substrates which are suitable for use as the mechanical support for the recording disc. It is also desirable that once a process to achieve targeted or predetermined magnetic properties is in place, minor independent adjustments to the magnetic properties can be readily made by means of relatively easy and straight forward changes in process parameters.
The magnetic recording film properties of cobalt-platinum (Co-Pt) alloys are known and have been reported to some extent. See, for example, the paper by M. Naoe et al., entitled "Preparation of High Coercivity Co-Pt Alloy Films by Targets-Facing Type of High Rate Sputtering," Intermag 1983, digest AC-5. These authors showed that a maximum coercivity of about 1600 Oe was achieved with a Co-Pt alloy in which the platinum content was of about 20 atomic percent. Their films were deposited on glass substrates with a dc magnetic field of about 125 Oe in the plane of the film. M. Yanagisawa et al., in a paper entitled "Corrosion-Resisting Co-Pt Thin Film Medium for High Density Recording," Intermag 1983, digest AC-6, describe sputtered Co-Pt thin films which showed similar results. A. J. Griest, Jr., in U.S. Pat. No. 3,755,796, discloses cobalt-platinum alloys for use in magnetic domain (magnetic bubble) applications but not in thin film form suitable for high density magnetic recording media.
For a magnetic material such as cobalt and its alloys to have a high coercive field, it is known that the crystal structure of a film thereof should be that of the hexagonal (HCP) phase with high magneto-crystalline anisotropy. As noted by Chen et al. in U.S. Pat. No. 4,202,932, it is well known that pure cobalt metal in the HCP phase at a low temperature will transform to a face-centered cubic (FCC) phase at higher temperatures. The transformation of the cobalt film to FCC phase causes a decrease in the coercive field to a point where it is no longer useful as a medium for high density recording. Lazzari et al. in their paper entitled "Thin Evaporative Films With High Coercive Force," published in the IEEE Transactions on Magnetics, September, 1967 (Vol. Mag-3, No. 3, Page 205) report that a perturbation of the structure and morphology of the magnetic material (cobalt) by the presence of a nonmagnetic underlayer of chromium, permits the attainment of coercivities of from 200 to 600 Oe., which may be adjusted by varying the thickness of the cobalt layer. These workers achieved their results by the evaporation of successive films of chromium and cobalt, and obtained a maximum value of coercivity with a cobalt film thickness of 200 .ANG. and the minimum with a cobalt film thickness of 1400 .ANG.. Similar results were reported by M. T. Maloney for sputter-deposited films of cobalt and chromium in a paper entitled "Sputtered Multilayer Films For Digital Magnetic Recording" IEEE Transactions on Magnetics, (Vol. 15, No. 3, July 1979 at Page 1135).
The prior art references identified hereinbefore deal principally with the basic magnetic properties of cobalt alloys. Nigh et al., in U.S. Pat. No. 4,079,169, do disclose magnetic recording media utilizing a nonmagnetic cobalt-based alloy as a protective cladding or isolation layer for an aluminum substrate. Their magnetic layer is iron or cobalt or an alloy of iron and cobalt which is vacuum deposited on the cobalt-based alloy and then topped by a protective layer of a metal such as rhodium. Nigh et al. also suggest that an undercoat layer of chromium or titanium may be deposited on the isolation layer prior to forming the magnetic layer.
Shine, in U.S. Pat. No. 4,221,615, teaches the use of a 50--50 atomic percent platinum-cobalt alloy to provide a soft magnetic material for use as the oscillating ball in flowmeters and the like. U.S. Pat. No. 4,154,875 to Yanagisawa et al. discloses a magnetic recording member comprising a magnetic layer of electroless plated cobalt-nickel-phosphorus formed on an electroless plated nickel-phosphorus alloy layer which is disposed on an aluminum base. The magnetic layer is then provided with a protective coating formed of a polysilicate. The aforementioned U.S. Pat. No. 4,202,932, to Chen et al. mentions a number of cobalt alloys (cobalt-rhenium, cobalt-ruthenium, cobalt-osmium, or mixtures thereof), for magnetic recording media, but no alloy involves cobalt and platinum.
Besides having acceptable magnetic recording properties, it is highly desirable for magnetic recording discs to possess excellent wear properties and be stable against oxidation of the magnetic layer. It has been customary to provide recording discs with special layers or coatings to protect the underlying layers against wear and contamination. See, for example, U.S. Pat. Nos. 4,152,487 and 4,154,875, to Yanagisawa and Yanagisawa et al., respectively, where the use of protective polysilicate films is disclosed. Likewise, Thomas et al., in U.S. Pat. No. 4,079,169, teach the use of a hard protective layer of a precious metal such as rhodium.
Yanagisawa et al, in their U.S. Pat. No. 4,069,360, also teach the use of an inorganic oxide layer coated with a lubricant layer to protect and reduce frictional wearing of both the surface of the recording disc and the surface of the magnetic recording head. The inorganic oxide film disclosed in this patent is a polysilicate (such as quartz, glass, silicate glass, borite glass, borosilicate glass, phosphate glass, or amorphous aluminum). The lubricant disclosed may be silicone oil, fluoric oil, fluorosilicone oil and a silane or a silazane group as a surface coupling agent.